Short for phase-locked loop, an electronic circuit that controls an oscillator so that it maintains a constant phase angle (i.e., lock) on the frequency of an input, or reference, signal. A PLL ensures that a communication signal is locked on a specific frequency and can also be used to generate, modulate and demodulate a signal and divide a frequency.

PLL is used often in wireless communications where the oscillator is usually at the receiver and the input signal is extracted from the signal received from the remote transmitter.